Wind turbines in HAWT design (horizontal axis) consist of four main parts as a structure, the base, the tower, the nacelle and the rotor with one or more blades.
The blades are mounted at fixed angularly spaced positions around the axis. The turbine includes a wind detection system which analyses the wind speed and direction repeatedly so as to repeatedly adjust the angle of the nacelle around a vertical axis of the tower, that is the angle of the rotor axis relative to the wind direction, and to adjust the angle of attack of the blades around the longitudinal axis of the blade relative to the wind speed.
A common target for structural measurements on wind turbine is to determine the deflection of rotor blades. This is either because the manufacturer wants to verify the original design or design improvements.
The setup of such a measurement is rather complicated and expensive (up to multiple $100,000) and time consuming. Typically this requires the application of strain gauges at predetermined positions along the length of the blade so that the deflection at leach location can be detected and analyzed.
Furthermore because of the expense of this method, testing is usually limited to one turbine without knowing if it is representative of multiple turbines. The conventional method is not suitable in a situation where the structural integrity of a blade is in question for example after lightning strikes.
During the operation of a wind turbine technical events such as structural failure (wear and tear), over loading, lightning strikes or flying debris in storm scenarios can create unsafe conditions. Most of those events can cause critical damage to the other third of the blade.
In those cases it is often imperative to verify the integrity of the structural strength of the blades structure to either insure the continued safe operation or to stop the operation of the turbine to prevent catastrophic failure scenarios such as blade parts falling off or a blade striking to the tower. The easiest way to verify structural integrity is to analyse and observe the visual blade deflection of all three blades which should be uniform under all load conditions.
Previous systems used and implemented for blade load measurements are for instance Bragg Fibres laminated into the blade structure during production. This allows observing and documenting blade load and deflection on several predetermined points along the blade axis. The use of Bragg fibres requires the installation during the initial production of the blade and cannot be applied at any later date. Bragg fibres also have shown a high percentage of degradation or failure after a just a few years in operation.
Other systems use Strain gauges which are typically applied inside the blade along the accessible inner 3rd of the blade length. Since it only can document load for the first 3rd of the blade length it is mostly unsuitable for detecting structural damage in the outer 3rd of the blade. While the process is very capable on observing blade moments at the blades root it is a very expensive application. Installation, instrumentation and post processing typical can account for expenses in excess of the cost of one or more blades. It is therefore not practical to proof or disproof structural anomalies with the objection to replace one blade, since the total costs would be in the range of a full rotor set of blades.
There have been also application attempts using laser deflectors inside blades. Those systems are either pre-installed during blade production or at a later point only accessing the inner 3rd of the blade due to inaccessibility of the blade. This system also has a number of technical restrictions. The blades inside typically has a number of structural support structures, like one or more shear webs or even chamber like structures which would block the line of sight. If one would assume a pre-installed system with a free line of sight blade core area, the system would have to overcome the look over or around an inner blade horizon. This horizon is created when the blade is bending under load in which case the blade axis near the tip and near the root can create angles in excess of 30 degree.
Another system uses one camera looking for a just one very short moment at a blade deflection relative to the tower. The system does not allow real time side by side blade deflection comparison and is using the least suitable blade position for a blade moving around the rotor disc and therefore it is not providing significant value.